The present invention relates to interference in a cellular communications system which employs frequency hopping.
One type of communications system is a cellular communications system. In a cellular communications system, the area over which service is provided is divided into a number of smaller areas called cells. Typically each cell is served from a base transceiver station (BTS) which has a corresponding antenna or antennas for transmission to and reception from a user station, normally a mobile station. Presently established cellular radio communications systems include GSM systems (Global System for Mobile Communications).
In a communications system, information is conveyed by means of a communications link which is divided into separate channels. In a digital cellular communications system, a plurality of channels occupy a single physical radio resource by virtue of a multiplexing technique called Time Division Multiple Access (TDMA). Time is divided into discrete periods called timeslots which are organised in time division frames, which are often called TDMA frames. In a GSM system each TDMA frame comprises the same number of timeslots, the number being 8. The timeslots are numbered sequentially. In GSM terminology, the timeslot numbers are 0 to 7 respectively. FIG. 1 shows timeslots numbered 0 to 7 arranged in 3 TDMA frames.
In a TDMA operated cellular communications system, the simplest way a channel is formed is by remaining on the same frequency band and allocating the same respective timeslot number of each consecutive time frame to that channel. This is shown in FIG. 2, where channel X is formed by repeatedly using timeslot number 0, and channel Y is formed by repeatedly using timeslot number 5. In the simple example of FIG. 2, all the TDMA frames shown are sent on the same RF carrier, i.e. at the same frequency.
In practical systems the number of cells means that frequency re-use is employed. In order to reduce interference in systems employing frequency re-use, it is known to employ frequency hopping. The same underlying basis of channel operation still applies when frequency hopping is being used, as will now be explained with reference to FIG. 3. In FIG. 3, three separate RF carrier frequencies are shown, namely F1, F2 and F3. Channel X is again formed by repeatedly using timeslot number 1 from consecutive TDMA frames, although now in this example such frames appear on different RF carriers according to a frequency hopping arrangement. Similarly, channel Y is again formed by repeatedly using timeslot number 5 from consecutive TDMA frames, with such frames appearing on different RF carriers according to the frequency hopping arrangement.
Within each timeslot a burst of one or more bits is transmitted and received.
Different BTSs will each carry out frequency hopping across their frequencies. The required hopping sequence for each subscriber unit, for example a mobile station (MS), is communicated to the MS as part of assignment signalling.
The frequency hopping arrangement or plan for a given BTS, or sector of a multi-sectored BTS, is determined by hopping determination parameters. For example, in the case of a GSM system, the frequency hopping arrangement is determined by a look-up table function defined in terms of (i) frame number (which is usually cycled, e.g. counted from 1 through to 4,000,000, say, and then started again from 1); (ii) hopping sequence number (an allocated number), and (iii) mobile allocation index offset (a predetermined offset).
Ideally, interference between different cells employing a same frequency or adjacent frequency due to frequency re-use would be entirely avoided by means of careful planning of the frequency re-use patterns and the frequency hopping arrangements selected. However, in practise, geographical factors, errors in planning, and changes to initial situations all lead to interference arising due to different cells transmitting on the same frequency, despite good intentions when planning the frequency re-use patterns. The effect on particular communications channels cannot be predetermined or resolved conventionally since each BTS can or will be operating with different values of hopping determination parameters, and moreover typically different BTSs will start their frame number count at different times so will be at different stages of their frame number count cycle.
The present invention addresses the problem of the above described interference.
According to one aspect of the present invention, there is provided a method of processing interference in a cellular communications system, as claimed in claim 1.
According to another aspect of the present invention, there is provided an apparatus for processing interference in a cellular communications system, as claimed in claim 8.
Further aspects of the invention are as claimed in the dependent claims.
The present invention advantageously provides a means for determining the effect of the hopping determination patterns of interfering cells.
Preferred versions of the present invention provide means for alleviating the interference.
Additional specific advantages are apparent from the following description and figures.